Switch-mode power supplies are used in a broad range of electronic applications, such as, for example, telecommunication systems, computers, television equipment, video equipment, etc., where it is important to accurately regulate a power supply output over a wide range of loads. Some advantages of switch-mode power supplies over other types of regulated power supplies (e.g., linear power supplies) are that they are smaller, weigh less, consume less power, and are highly efficient compared to many other types of regulated power supplies.
In numerous applications, switch-mode power supplies must operate with stability over a very wide range of loads. One technique used for operating such switch-mode power supplies over very wide ranges of loads is to fold back the switching frequency in response to decreases in the loads. However, in certain applications, the switch-mode power supply may be operated as a current source instead of as voltage source. Stability in this mode of operation is much more difficult to achieve.
Valid loads for voltage source switch-mode power supplies typically range from some non-zero minimum to some predetermined maximum. Short circuits are typically handled by turning off the power supply and retrying to start periodically. Valid loads for power supplies operating in current source mode range from open circuit to short circuit. The power supply is not permitted to power cycle and must provide a stable current/voltage at all loads. Typically, current source mode power supplies have a maximum output voltage they will put out at open circuit. This protects the power supply components from excessive voltages. The conditions of open circuit and short circuit represent opposite ends of the load spectrum, but both conditions are very light loads on a current source power supply. Maximum load occurs halfway in between where the output voltage and current are at their maximum.
One technique used to increase stability over a very wide load range is to lower the power supply switching frequency at light loads. An existing technique used to perform this frequency fold-back in switch-mode power supplies is to add a winding to the power transformer that is ratioed to the output voltage of the supply involved. This voltage is then used as an input to a voltage controlled oscillator. This technique can be used readily with a voltage source, where the output voltage varies linearly from minimum load to maximum load. However, this added winding frequency fold-back technique cannot be used with a current source, because the minimum load condition for a current source occurs at both ends of the load spectrum: full output voltage and no output voltage.
As such, another technique was derived to perform frequency fold-back with a current source mode switch-mode power supply. Every switch-mode power supply has at least one error amplifier to sense the value of the output and compare it to a reference value. The result of this comparison is commonly referred to as the compensation signal. The compensation signal is proportional to the output load in both current source mode and voltage source mode power supplies. This technique takes this compensation signal, scales it, and uses it to control the power supply's oscillator. The lower frequency at lighter loads improves the stability of the control loop under these conditions.
Notwithstanding the distinct advantages of the above-described technique, it would be advantageous to have a switch-mode power supply that is even more stable than prior switch-mode power supplies. As described in detail below, the present invention provides an improved switch-mode power supply that is more stable than prior switch-mode power supplies.